Mammary carcinogenesis is a multistep process that requires the concerted action of multiple genes. These genes mediate several different morphogenetic transformation pathways characterized by partially- transformed protoneoplastic lesions. The long-term goal of the proposed research is to define the role of protooncogenes in this system. Our strategy is to (i) isolate and characterize new mammary protooncogenes identified by mouse mammary tumor virus (MMTV) insertion mutation, (ii) analyze mammary protooncogenes in rodent and human protoneoplasias, tumors, and cell culture systems, and (iii) reconstruct postulated transformation pathways using our transgenic mammary gland system. In the initial grant period, we propose to use our unique combination of molecular, biological, and clinical resources to (i) characterize mammary protooncogene expression vectors in transgenic mammary glands constructed in the MMTV-free WLC-0 mouse strain, (ii) isolate and characterize new protooncogenes activated by MMTV provirus insertion using mammary protoneoplasias and tumors from the WXG-2 mouse strain, and (iii) isolate and characterize human homologs of the new mammary protooncogenes identified in the mouse system and determine if these genes are also mutated during mammary carcinogenesis in humans.